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Abstract

Our study of fluid and melt inclusions in quartz and feldspar from granite pegmatite from the Precambrian Rønne granite, Bornholm Island, Denmark revealed extremely alkali bicarbonate- and carbonate-rich inclusions. The solid phases (daughter crystals) are mainly nahcolite [NaHCO3], zabuyelite [Li2CO3], and in rare cases potash [K2CO3] in addition to the volatile phases CO2 and aqueous carbonate/bicarbonate solution. Rare melt inclusions contain nahcolite, dawsonite [NaAl(CO3)(OH)2], and muscovite. In addition to fluid and melt inclusions there are primary CO2-rich vapor inclusions, which mostly contain small nahcolite crystals. The identification of potash as a naturally occurring mineral would appear to be the first recorded instance. From the appearance of high concentrations of these carbonates and bicarbonates we suggest that the mineral-forming media were water and alkali carbonate-rich silicate melts or highly-concentrated fluids. The coexistence of silicate melt inclusions with carbonate-rich fluid and nahcolite-rich vapor-inclusions indicates a melt-melt-vapor equilibrium during the crystallization of the pegmatite. These results are supported by results of hydrothermal diamond-anvil cell experiments in the pseudoternary system H2O - NaHCO3 - SiO2. Additionally, we show that boundary layer effects were insignificant in the Bornholm pegmatites, and are not required for the origin of primary textures in compositionally simple pegmatites at least.

Item Type:	Article
Authors/Creators:	Thomas, R and Davidson, P and Schmidt, C
Keywords:	pegmatites, fluid and melt inclusions, molten carbonates, carbon dioxide, HDAC experiments
Journal or Publication Title:	Contributions to Mineralogy and Petrology
ISSN:	0010-7999
DOI / ID Number:	https://doi.org/10.1007/s00410-010-0533-z
Additional Information:	The original publication is available at www.springerlink.com
Item Statistics:	View statistics for this item

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